Models for the treatment of crystalline solids and surfaces

Electronic Structure of Small Isolated and Supported Manganese Oxide Clusters

In the present work, possible molecular models of the isolated manganese oxides and supported Mn3Ox/Al2O3 structures were built based on small clusters of passivated MnOx. The support was represented as a simplified model of the alumina tetramer cluster based on small fragments of AlOxHy. Combinations of MnOxHy and AlOxHy clusters were made to form both the isolated and supported manganese oxides clusters. The electronic structure of these systems was characterized by ab initio methods (DFT and CASPT2). It was observed that the vertical excitation energy of the isolated and supported Mn3OxHy clusters is significantly lower than that of the alumina cluster model, while both the isolated and supported Mn3OxHy wave function characters are qualitatively similar with respect to the ground state and electronic transition processes, suggesting that the alumina cluster behaves as an inert support, since there is little contribution of this component in the description of the low-lying electronic states. The present study also reports for the first time the spectroscopic parameters of several clusters containing the manganese transition metal atom.

Periodic Bootstrap Embedding

Bootstrap embedding (BE) is a recently developed electronic structure method that has shown great success at treating electron correlation in molecules. Here, we extend BE to treat surfaces and solids where the wave function is represented in periodic boundary conditions using reciprocal space sums (i.e., k-point sampling). The major benefit of this approach is that the resulting fragment Hamiltonians carry no explicit dependence on the reciprocal space sums, allowing one to apply traditional nonperiodic electronic structure codes to the fragments even though the entire system requires careful consideration of periodic boundary conditions. Using coupled cluster singles and doubles (CCSD) as an example method to solve the fragment Hamiltonians, we present minimal basis set CCSD-in-HF results on 1D conducting polymers. We show that periodic BE-CCSD can typically recover ∼99.9% of the electron correlation energy. We further demonstrate that periodic BE-CCSD is feasible even for complex donor-acceptor polymers of interest to organic solar cells─despite the fact that the monomers are sufficiently large that even a Γ-point periodic CCSD calculation is prohibitive. We conclude that BE is a promising new tool for applying molecular electronic structure tools to solids and interfaces.

Electronic structure and physicochemical properties of the metal and semimetal oxide nanoclusters

Clusters are physical entities composed of a few to thousands of atoms with capabilities to develop novel materials, like cluster-assembled materials. In this sense, knowing the electronic structure and physicochemical properties of the isolated clusters can be useful to understand how they interact with other chemical species by intermolecular forces, as free, embedded, and saturated clusters, and by intramolecular forces, acting as support clusters. In this way, in the present work, the electronic structure and physicochemical properties of metal oxide nanoclusters (MgO, Al₂O₃, SiO₂, and TiO₂) were studied by highly correlated molecular quantum chemistry methods. Through the electronic state's characterization, a semiconductor aspect was found for the titania oxide nanocluster (T_e < 0.8 eV) while the other agglomerates showed a characteristic of insulating material (T_e > 3.3 eV). From the stability index, the following stability order can be characterized: (SiO₂)₄ > (Al₂O₃)₄ > (MgO)₄ > (TiO₂)₃. Initial information of intermolecular and intramolecular forces caused by the studied clusters was calculated through the relative electrophilicity index, which classified the (MgO)₄ and (TiO₂)₃ clusters as the more reactive ones, in which the (MgO)₄ cluster was identified as a nucleophilic species, while the (TiO₂)₃ cluster as an electrophilic molecule.

Full Configuration Interaction Quantum Monte Carlo treatment of fragments embedded in a periodic mean field

We present an embedded fragment approach for high-level quantum chemical calculations on local features in periodic systems. The fragment is defined as a set of localized orbitals (occupied and virtual) corresponding to a converged periodic Hartree-Fock solution. These orbitals serve as the basis for the in-fragment post-Hartree Fock treatment. The embedding field for the fragment, consisting of the Coulomb and exchange potential from the rest of the crystal, is included in the fragment's one-electron Hamiltonian. As an application of the embedded fragment approach we investigate the performanceof full configuration interaction quantum Monte Carlo (FCIQMC) with the adaptive shift. As the orbital choice we use the natural orbitals from the distinguishable cluster method with singles and doubles. FCIQMC is a stochastic approximation to the full CI method and can be routinely applied to much larger active spaces than the latter. This makes this method especially attractive in the context of open shell defects in crystals, where fragments of adequate size can be ratherlarge. As a test case we consider dissociation of a fluorine atom from a fluorographane surface. This process poses a challenge for high-level electronic structure models as both the static and dynamic correlations are essential here. Furthermore the active space for an adequate fragment (32 electrons in 173 orbitals) is already quite large even for FCIQMC. Despite this, FCIQMC delivers accurate dissociation and total energies.

Large Differences in the Optical Spectrum Associated with the Same Complex: The Effect of the Anisotropy of the Embedding Lattice

Transition-metal complexes with a well-defined geometry are usually considered to display almost the same properties independently of the system where they are embedded. Here we show that the above statement is not true depending on the anisotropy of the host lattice, which is revealed in the form of the electric field created by the rest of lattice ions over the complex. To illustrate this concept we analyze the origin of the surprisingly large differences in the d-d optical transitions of two systems containing square-planar CuF₄^2- complexes, CaCuF₄, and center II in Cu²⁺-doped Ba₂ZnF₆, even though the Cu²⁺-F^-distance difference is just found to be 1%. Using a minimalist first-principles model we show that the different morphology of the host lattices creates an anisotropic field that red-shifts the in vacuo complex transitions to the 1.25-1.70 eV range in CaCuF₄, while it blue-shifts them to the 1.70-3.0 eV region in Ba₂ZnF₆:Cu²⁺. This particular example shows how the lattice anisotropy strongly alters the optical properties of a given transition-metal complex. This knowledge opens a new path to tune the spectra of this large family of systems.

Numerical investigation of the elastic scattering of hydrogen (isotopes) and helium at graphite (0001) surfaces at beam energies of 1 to 4 eV using a split-step Fourier method

We report simulations of the elastic scattering of atomic hydrogen isotopes and helium beams from graphite (0001) surfaces in an energy range of 1–4 eV. To this aim, we numerically solve a time-dependent Schrödinger equation using a split-step Fourier method. The hydrogen- and helium-graphite potentials are derived from density functional theory calculations using a cluster model for the graphite surface. We observe that the elastic interaction of tritium and helium with graphite differs fundamentally. Whereas the wave packets in the helium beam are directed to the centers of the aromatic cycles constituting the hexagonal graphite lattice, they are directed toward the rings in case of the hydrogen beams. These observations emphasize the importance of swift chemical sputtering for the chemical erosion of graphite and provide a fundamental justification of the graphite peeling mechanism observed in molecular dynamics studies. Our investigations imply that wave packet studies, complementary to classical atomistic molecular dynamics simulations open another angle to the microscopic view on the physics underlying the sputtering of graphite exposed to hot plasma.

Advances in correlated electronic structure methods for solids, surfaces, and nanostructures

Calculations of the electronic structure of solids began decades ago, but only recently have solid-state quantum techniques become sufficiently reliable that their application is nearly as routine as quantum chemistry is for molecules. We aim to introduce chemists to the pros and cons of first-principles methods that can provide atomic-scale insight into the properties and chemistry of bulk materials, interfaces, and nanostructures. The techniques we review include the ubiquitous density functional theory (DFT), which is often sufficient, especially for metals; extensions such as DFT + U and hybrid DFT, which incorporate exact exchange to rid DFT of its spurious self-interactions (critical for some semiconductors and strongly correlated materials); many-body Green's function (GW and Bethe-Salpeter) methods for excited states; quantum Monte Carlo, in principle an exact theory but for which forces (hence structure optimization and dynamics) are problematic; and embedding theories that locally refine the quantum treatment to improve accuracy.

Structure of Ag Clusters Grown on Fs-Defect Sites of an MgO(1 0 0) Surface

The structure of AgN clusters (N=1-4, 6, 8, 10), both in the gas phase and grown on the MgO(1 0 0) surface containing Fs-defects, has been investigated by a density functional basin-hopping (DF-BH) approach. In analogy with what observed in the case of gold clusters, it is found that the presence of the defect implies a double frustration and a cylindrical invariance of the metal-surface interaction, causing small Ag clusters growing around the Fs defect to be highly fluxional. Nevertheless, two different structural crossovers are found to be induced by the metal-defect interaction for the adsorbed clusters such that: 1) planar structures prevail for N<or=4 (as in the gas phase); 2) noncrystalline (fivefold symmetric) structures, which are the lowest energy ones in the gas phase for medium sized AgN clusters (N>or=7), prevail for N=6 and N=8; 3) distorted face-centered cubic (fcc) structures grown pseudomorphically on the defected surface prevail for N=10. The transition from fivefold to fcc motifs is rationalized in terms of the double-frustration effect, which increases the bond strain of the noncrystalline structures. Detrapping energies from the defect were also calculated; the lowest energy pathway corresponds to the detachment of a dimer.

Electrostatic properties of adsorbed polar molecules: opposite behavior of a single molecule and a molecular monolayer

We compare the electrostatic behavior of a single polar molecule adsorbed on a solid substrate with that of an adsorbed polar monolayer. This is accomplished by comparing first principles calculations obtained within a cluster model and a periodic slab model, using benzene derivatives on the Si(111) surface as a representative test case. We find that the two models offer diametrically opposite descriptions of the surface electrostatic phenomena. Slab electrostatics is dominated by dipole reduction due to intermolecular dipole-dipole interactions that partially depolarize the molecules, with charge migration to the substrate playing a negligible role due to electric field suppression outside the monolayer. Conversely, cluster electrostatics is dominated by dipole enhancement due to charge migration to/from the substrate, with only a small polarization of the molecule. This establishes the important role played by long-range interactions, in addition to local chemical properties, in tailoring surface chemistry via polar molecule adsorption.

ATR-FTIR measurements and quantum chemical calculations concerning the adsorption and photoreaction of oxalic acid on TiO2

The adsorption and photoreaction of oxalic acid on the surface of anatase and rutile TiO2 nanoparticles have been studied using a combined experimental and theoretical approach. In the dark, the experimental adsorption reaches an equilibrium state that can be described as a mixture of adsorbed water and oxalic acid molecules, with the latter forming two different surface complexes on anatase and one on rutile particles. When the system is subsequently illuminated with UV(A) light, the surface becomes enriched with absorbed oxalic acid, which replaces photo-desorbed water molecules, and one of the adsorbed oxalic acid structures on anatase is favoured over the other.

Effect of dehydration on sulfate coordination and speciation at the Fe-(hydr)oxide-water interface: a molecular orbital/density functional theory and Fourier transform infrared spectroscopic investigation

The effect of dehydration on the coordination and speciation of sulfate at the Fe-(hydr)oxide-H2O interface was investigated using molecular orbital/density functional theory (MO/DFT) and Fourier transform infrared (FTIR) spectroscopy. IR frequency calculations were performed at the UB3LYP/6-31+G(d) level of theory for potential sulfate (bidentate bridging, monodentate, and H-bonded) and bisulfate (bidentate bridging and monodentate) surface complexes. MO/DFT calculated IR frequencies were compared to available IR literature results and attenuated total reflectance (ATR) FTIR spectra collected in our laboratory of sulfate adsorbed at the hematite-H2O interface. IR frequency calculations performed using the larger 6-311+G(d,p) basis set resulted in minor frequency shifts that did not dramatically alter the agreement with experiment. This investigation proposes that sulfate undergoes a speciation change as a function of surface dehydration. A generalized model for the speciation change is proposed as follows. (1) At the Fe-(hydr)oxide-H2O interface, sulfate adsorbs as a bidentate bridging or monodentate surface complex under most experimental conditions. (2) Upon surface dehydration, sulfate changes speciation to form bidentate bridging and/or monodentate bisulfate. However, surface dehydration does not yield 100% speciation change but leads to a mixture of sulfate and bisulfate. (3) The speciation change is reversible as a function of rehydration. The reversibility of the sulfate-bisulfate speciation change is chiefly determined by the local hydration environment of the O-H bond in bisulfate. Under dehydrated conditions, the O-H bond length is approximately 0.98 A. The bond length substantially increases (bond strength decreases) to approximately 1.03 A when the initial H-bond network is re-established through hydration, likely leading to deprotonation upon full mineral surface hydration.

Structural and Electronic Properties of Li2B4O7

The reliability of various quantum-chemical approaches for the calculation of bulk properties of lithium tetraborate Li(2)B(4)O(7) was examined. Lattice parameters and the electronic structure obtained with density-functional theory (DFT), with DFT-Hartree-Fock (HF) hybrid methods, and with the semiempirical method MSINDO were compared to available experimental data. We also compared the results at DFT level using different wave functions, either based on linear combinations of atom-centered orbitals (LCAO), or on plane waves, as implemented in the crystalline orbital programs CRYSTAL and VASP. The basis set dependence of calculated properties was investigated for the LCAO method. In the plane wave approach ultrasoft pseudopotentials (US PP), and projector-augmented wave (PAW) potentials were used to represent the core electrons. For all methods under consideration, the calculated Li(2)B(4)O(7) structure parameters are close to each other and agree within a few percent with measured values. A more pronounced method dependence was found for the band structure, the band gap and the cohesive energy. Closest agreement between theoretical and experimental results for the band gap was obtained with the DFT-HF hybrid methods while pure DFT methods underestimate and HF based methods overestimate the measured value. It was found that the calculated band gap strongly depends on the atomic basis set in the LCAO approach. The description of the core electrons considerably affects the cohesive energy obtained with the plane wave approach. Atomic charges based on a Mulliken analysis were compared to effective charges obtained from Raman spectroscopy. Electron density maps are used to analyze the character of B-O and Li-O interactions.

Toward an Understanding of the Formation of Vanadia−Titania Catalysts

Structures of hydrated vanadia species on the TiO2-anatase surfaces were investigated using the semiempirical molecular orbital method MSINDO. The (101), (001), and (100) surfaces of anatase were considered. They were modeled by appropriate two-dimensional cyclic clusters of TiO2. Monomeric and dimeric hydrated vanadia species on the anatase surfaces were simulated by adsorbing VO4H3 and V2O7H4 molecules, respectively. Different adsorption structures were considered, and their stabilities at 300 and 600 K were tested by constant-temperature Born-Oppenheimer molecular dynamics simulations in the framework of MSINDO. Structural features of the vanadia-titania catalysts found in extended X-ray absorption fine structure, secondary ion mass spectrometry, IR, Raman, and NMR spectroscopy and conductivity experiments can be explained by the present calculations.

Properties of Ternary Insulating Systems: The Electronic Structure of MgSO4·H2O

Structural and electronic properties of (100)-oriented MgSO(4) and MgSO(4).H(2)O surfaces and the adsorption of water on the latter were investigated theoretically with a combination of ab initio and semiempirical methods. Ab initio electronic structure calculations were based on a density functional theory (DFT)-Hartree-Fock (HF) hybrid approach. The semiempirical method MSINDO was used for the determination of the local adsorption geometry of the water molecule. With the hybrid method good agreement was obtained with the experimental band gap of 7.4 eV determined with electron energy loss spectroscopy of polycrystalline MgSO(4).H(2)O samples under ultrahigh vacuum conditions. The valence bands of the (100) surfaces of both MgSO(4) and MgSO(4).H(2)O are formed mainly by the O2p levels, whereas the S2p states contribute to the lower part of the conduction band. The preferred adsorption site of water at MgSO(4).H(2)O (100) is above a surface Mg atom. The water molecule is stabilized by two additional hydrogen bonds with surface atoms. Only small differences between the electronic structure of MgSO(4).H(2)O and MgSO(4) were observed. Also, the molecular adsorption of water on the MgSO(4).H(2)O surface leads to only small shifts of the electronic energy levels.